Menthol is known as an essential oil, because its vapours have an aromatic “essence”. The taste and smell of menthol is pleasant and it offers relief from symptoms of colds or flu [1 Paul I M, Beiler J S, King T S, et al. Vapor Rub, Petrolatum, and No Treatment for Children with Nocturnal Cough and Cold Symptoms. Pediatrics 2010; 126(6):1-8.]. Many products for administering essential oils incorporate combinations of essential oils that include menthol and are for topical use. Usually topically applied products comprising of menthol in an oil base are semisolid at ambient temperature. An example of such a product is Vics VapoRub®.
U.S. Pat. No. 7,138,394 [Schwarz and Welsspapir] teaches the use of a composition for topical delivery of non-steroidal analgesic cream medication that comprises among its ingredients menthol together with camphor and medium chain triglyceride (MCT). This mixture is a semisolid at room temperature. Solubilization of the non-steroidal analgesic in the topical cream was accomplished with the use of medium-chain triglyceride along with the combination of menthol and camphor. Semisolid products and creams do not lend themselves to accurate measurement of the amount of compound used or applied.
Menthol is also commonly used as a component of cold remedies that are ingested orally. One example is a group of products marketed under the brand name Halls®. These are typically packaged as wrapped, hard-candy-lozenges. These lozenges contain as their therapeutic dose as much as 10 mg menthol each. One of these products includes among its ingredients MCT, Halls Mountain Berry with Soothing Honey Center (Active ingredient: Menthol 2.5 mg. Inactive ingredients: citric acid; cottonseed oil; elderberry juice; flavors; ginger; glucose syrup; glycerin; honey; lemon grass; MCT oil; sage; soy lecithin; sucrose; water; white thyme).
A review conducted by the Federeal Drug Administration (FDA) in the United States concluded that the dose of menthol required for antitussive (i.e., cough-suppressant) effect is at least 5 mg [Federal Register/Vol 52, No. 155/Wednesday, Aug. 12, 1987, page 39946]. Liquid products for the treatment of cough are generally in a syrup form and contain menthol as a therapeutically active agent in the dose range of 5-20 mg per dose, with each dose provided in the general volume of a spoonful (5 mL). An example of such a product is Buckley's Mixture®. The liquid oral preparations containing menthol are water-based syrups, which necessitates the inclusion of solublilizers that keep the menthol in the liquid phase.
Pure menthol is solid at ambient temperature, with a melting point of 42 degrees Centigrade. Menthol is poorly soluble in water-based preparations, but it is readily dissolvable at high concentration in alcohol. Menthol is also soluble in oils, including MCT. However, at higher concentrations, these solutions are often semisolid and waxy at ambient temperature (18-23° C.). Moreover, as will be discussed hereinbelow, these oil formulations typically exhibit a temperature hysteresis effect that normally mitigates their use for administration of a liquid menthol composition.
Further, among the difficulties associated with the prior art approaches to the use of menthol are that many of the products are semisolid and are suitable only for topical application, or for inhalation. Semisolid preparations are not suitable for oral use because they cannot be measured reliably. As such, because of their tendency to be semisolid, waxy or ointment-like, preparations of menthol in oil are used for topical application to the skin.
Often these products are added to a vaporizer and they are not for oral use, because the magnitude of an oral dose must be obtainable by means that are simple, accurate and reliably dispensed. Other menthol products are in the style of confectionaries, or menthol is present as one component of conventional bad-tasting cough syrup that needs to be administered by the spoonful.
Available through the internet are other unspecified liquid menthol products of unspecified concentration, and which some persons use to add menthol to cigarettes.
Additionally, one alternative recipe for a menthol product, is to dissolve two volumes of menthol crystals in one volume ethanol (http://nu-vapor.com/forum/diy-liquids/1604-diy-menthol-drops-make-your-own.html). The difficulty with this kind of product is the unreliable nature of ethanol, which is volatile, and hence results in an unreliable concentration of menthol that is influenced by the amount of ethanol evaporation occurring over time. As alcohol evaporates with storage, the concentration of menthol will eventually reach a sufficiently elevated level to cause solidification.
Water-based menthol preparations are commonly available, but of necessity, are relatively dilute and typically require the consumption of at least 1 teaspoon-full of liquid.
Alternatively, Tonori et al [EP1640022 A1] teach that MCT is useful for dissolving menthol in a range of 0.5- to 10-fold by weight. However, his technique requires preparation of the composition by heating to 80° C. and then emulsification of the mixture to prepare a menthol-containing emulsion. The composition described by Tonori et al was used to provide an L-menthol emulsion in a fat or oil together with water and a surfactant to be used during endoscopic surgery to inhibit contraction of the digestive tract. The Tonori et al patent does not disclose anything about the solidification behaviors of menthol in MCT and offers no insight as to the suitability of a liquid menthol composition for use as a liquid cough suppressant.
As such, there exists no product suitable for the mainstream drug-store market that provides a way to provide a concentrated liquid solution of menthol in a manner that makes it possible to obtain a therapeutic dose (about 5-20 mg menthol) in one or two drops of liquid.
Such a product, if currently available, would have a broad appeal over a wide variety of consumer ages.
However, although menthol is a liquid oil when it is above 42° C., and is soluble in oil, the goal of a meaningful dose of menthol in a single drop of oil has remained problematic. The greatest difficulty is that if the menthol product becomes semisolid, it is useless for oral consumption unless reheated to a higher temperature.
Accordingly, there is a need in the art for a non-alcoholic, non-water-based, liquid menthol product that provides a consistently reproducible therapeutic dose and that returns to liquid at room temperature, (e.g. 18-23° C.), even after having been previously solidified at a lower temperature, which would be expected to occur during regular transport or storage.
Also preferred, would be a product suitable for administration by means of dispensing preferably one single drop accurately and reliably.
As such, to overcome the difficulties of the prior art, it would be advantageous to provide a liquid menthol composition that would be suitable for administration as a liquid material, and more preferably, to provide a liquid menthol composition for administration in a droplet form.
Accordingly, it would advantageous to provide a liquid menthol composition which would obviate or mitigate at least one of the above-mentioned disadvantages of the prior art.
Further, it would also be an advantage of the present invention to provide a novel composition that is useful as a medication to alleviate the symptoms of colds or flu.